In a mobile system, several transmitting antennas are employed to transmit user information in the downlink direction. In the uplink direction the use of several antennas will probably become relevant with the new media terminals. Use of a plurality of transmit antenna branches e.g. in a mobile system base station provides diversity gains over the radio connection. Diversity gains are obtained e.g. by means of coherent signal combination and more effective prevention of fading.
The values a signal obtains over the radio connection are modelled by means of complex numbers, where the absolute value or module of the complex number represents signal amplitude and the polar angle or argument of the complex number represents signal phase. The signals transmitted via different antennas are summed in the combined signal propagating over the radio path, the complex values of the signal thus corresponding to the sums of the values obtained by the signals transmitted via different antennas. The set of values the combined signal has obtained in the complex plane is referred to as a combined constellation. A combined constellation is utilized in the receiver for deducing, from the complex value of the combined signal, the signals transmitted from different antenna branches of the transmitter and combined over the radio path to provide the received complex value. In several cases the received complex value does not fully correspond to any constellation point, and the receiver must therefore make a decision for example on the basis of the constellation point that is closest to the received complex value. As the receiver moves in the radio system, the combined constellation is also continuously altered since the transmission channels change e.g. due to different propagation paths of the signals.
The prior art arrangements are able to react to the changing values or locations of the constellation points in the complex plane. However, in these arrangements the bit-to-symbol mapping remains constant during the connection, i.e. mapping of bits to symbols is independent of the channel status. Since the bit-to-symbol mapping is fixed and radio channel statuses keep changing, the likelihood of erroneous bit decisions made by the receiver increases, thus reducing the accuracy of identification of the transmitted information in the receiver.